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Old 26-01-2022, 03:20 PM
Rod-AR127 (Rod)
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Help from the Brains Trust - Stated Vs Actual FL and help positioning the new focusse

Hi, me again
I have the Bresser AR127L achromatic telescope which is supposed to have a focal length of 1200mm, making it an F9.44.
However, the measurement from the front of the objective lens to the furthest end of the focusser racked fully out is only 1100mm.
Am l missing 100mm, is the last 100mm from the star diagonal up to the bottom element in the EP, is 1200mm just and advertising lie or am l missing / not understanding something here?

I'm in the process of changing focusser from the 2.5" Hexafoc to a Bintel/GSO Crayford and l need to get the new focuser in the correct position. Given the new focusser has twice the movement range I'm confident l can put it roughly in the same position as the old one, maybe even a bit closer, but l need to ensure I'm not cutting off any of the focussed light due to the difference in internal aparture of the new focusser- see photo of old and new focusser below. Old on top, new bellow.

In my head l am seeing the objective lens as the flat base of the triangle and the focal point being the pointy top of the triangle, therefore l can determine the required internal aparture required at any point in the light path to ensure l am not cutting off any of the light.
So with my 1100mm measurement Vs 1200mm stated and the smaller internal aparture of the new focuser, I'm a bit stuck.
I plan to have an adaptor made to fit the new focusser to OTA.

Original focusser 45mm range, 140mm overall length racked in, 65mm internal aparture.

New focusser 80mm range, 145mm overall length racked in, 50mm internal aparture.

Any help greatly appreciated.

Rod.
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Last edited by Rod-AR127; 26-01-2022 at 03:33 PM.
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Old 26-01-2022, 03:39 PM
glend (Glen)
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Assuming your using it visually, both a diagonal mirror and the eye piece will add focal length components. Each EP will have a different focal point. The published 1200mm simply refers to the distance to the focal point. Most refractors designed for visual use will have adequate racking range to suit EP focal lengths.
Now I have, in the past owned a Bresser 150mm f5 with that Hex focuser, and I even used it for narrowband imaging when I started with imaging. It certainly was capable of supporting a camera and filter wheel without issue. My point being, a focuser upgrade on that Bresser is not necessary, imho, and likely a case of over capitalising the scope, ie, you will never get a higher resale value when you eventually sell it.
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Old 26-01-2022, 03:54 PM
Rod-AR127 (Rod)
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Yes, purely for visual.
I just drew the 1200mm triangle on the floor and my internal aparture of 50mm is 720 mm from objective so no issues with the new focusser cutting down light.
Sometimes you just need to verbalise or, in this case, write things down for it all to become clear.

My hexafoc is damaged and this new Bintel Crayford style was one of the very few options available without selling a kidney, noting l still have to get an adaptor made up.

Thanks for the confirmation on FL Glen, sets my mind (and wallet) at ease. I've done a LOT of googling trying to figure this one out.
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Old 26-01-2022, 04:00 PM
JA
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Hi Rod,

To determine the focal point and hence the focal length, reinstall the old focuser, since it fits the scope tube, and then take a piece of vellum/tracing paper or cooking parchment/wax paper and some how mount it across the end of the focuser as if it were a lens cap (perhaps use tape) and then point the scope during daytime at an object at infinity and bring the scope to focus on the tracing paper surface. You now have your focal point and can have a measure of the focal length, bearing in mind that it's measured from somewhere within the optics. You can also make a determination from the field of view, but not as accurately.

Best
JA
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Old 26-01-2022, 04:04 PM
Rod-AR127 (Rod)
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Interesting, will do that, thanks.
Rod.
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